Conventional manufacturing processes for soldering through-hole components with fine pitch on a printed circuit board can be classified into the following five types. The first one is the print process of solder paste via reflow soldering. The second is the wave soldering process via carrier. The third is the iron soldering process. The fourth is the automatic point-soldering process. The last is the numerical control (NC) multi-point soldering process. The disadvantages of these prior art processes are as follows.
The first type of process does not provide solder sufficiently to each pin of the through-hole component during the soldering process. By the first type of process, therefore, the through-hole component cannot withstand a large impact force from outside. In the second type of process, the printed circuit board is disposed at an additional height when being placed on the carrier. The additional height leads to the hard-to-solve problems of solder bridges and "opens" during the process. The "opens" are conditions in which the pins of the through-hole component get no molten solder when being soldered. Besides, the second type of process needs a large number of carriers for mass production, and thus is expensive. The third type of process consumes much time and also brings a great quantity of residual flux to the through-hole component. In a circuit test for the printed circuit board through the third type process, the residual flux will lead to an incorrect test result. In the fourth type of process, a control program is necessary for the automatic process. In addition, the soldering speed of the fourth type of process is very slow. The fifth type of process brings a large number of solder bridges to the through-hole component.
FIGS. 1(A), 1(B) and 1(C) show the process and configuration of a prior art multi-point soldering machine. A printed circuit board 3 is manufactured with a double-side board process. After surface mount devices (SMD) 2 are mounted, through-hole components 1 are placed over corresponding holes on the printed circuit board 3 either manually or by a machine. Pins 4 of the through-hole components 1 are sprayed with flux. After being preheated, the printed circuit board 3 is placed on the multi-point soldering machine for the soldering.
The multi-point soldering machine includes a solder bath tank in which the level of the solder wave 5 is controlled by a motor. The multi-point soldering machine also includes a fixture 6 having a plurality of openings 7. Each opening 7 corresponds to one of the through-hole components 1. The location of each opening 7 depends on the location of the corresponding through-hole component 1. Before the soldering, the printed circuit board 3 is first placed on the fixture 6, as shown in FIG. 1(A). At the beginning of soldering, a solder wave 5 is controlled to rise up into each opening 7, and the solder wave 5 wets the pins 4 of the corresponding through-hole component 1, as shown in FIG. 1(B). After the molten solder stays on the pins 4 for a certain period, the solder wave 5 falls, as shown in FIG. 1(C).
For fine-pitched through-hole component 1, this multi-point soldering process cannot avoid solder bridges 32. There are two reasons for which solder bridges are produced during this process. First, the solder wave 5 cannot pull down the solder bridges 32 between the adjacent pins 4 due to the high surface tension of the solder, when descending. Next, the solder bridge 32 is easily formed between adjacent ones of the pins 4 due to the formation of a large quantity of oxide over the solder, which inherently increases the surface tension of the solder.
In view of these disadvantages of the traditional technique, the main objective of the invention is to provide a soldering method to improve the yield, to shorten the cycle time, to enhance the reliability of soldered joints, and to reduce the cost. Another object of the invention is to overcome the problem of a great quantity of solder residuals such as solder bridges formed in the multi-point soldering process. A hot gas, mentioned in this invention, represents comprehensively a hot air or a hot inert gas.